The present invention relates to a power supply voltage monitoring circuit and, more particularly, to a power supply voltage monitoring circuit suited to be incorporated in a semiconductor integrated circuit device which is operable by having the potentials of power supply voltages switched in two steps.
Generally, a power supply voltage monitoring circuit is a circuit for producing an alarm or the like or stopping the operation of a related circuit when the supply voltage becomes lower (or higher) than a predetermined value thereof.
Standard power supply voltages used in various electronic devices now available are, for example, 5 V, 9 V, 12 V, 24 V, .+-.15 V, -5.2 V and, for purposes of making a wider range of use, it is desirable for such semiconductor devices to be so designed from the outset that they can be commonly used with two or more power supply systems whose supply voltages are different from one another. In order to incorporate such a supply voltage monitoring circuit in a semiconductor integrated circuit device which is capable of being used with a plurality of power supply systems, conventionally it was necessary to use either one of the following two methods:
i) The setting of the monitoring voltage in the power supply voltage monitoring circuit be effected externally of the semiconductor device and such setting be specific to the particular electronic device to be used; or PA1 ii) A plurality of the power supply voltage monitoring circuits be incorporated in the device and one of them to be used be selected from external means. PA1 a potential power supply line; PA1 a reference voltage source connected at its one side to the low potential power supply line; PA1 a first comparing means having a threshold value at a supply voltage being an intermediate voltage between the high and low power supply voltages; PA1 a second comparing means having a comparator receiving the voltage to be compared against a reference voltage supplied from the reference voltage source; and PA1 a switching means for changing in response to an output from the first comparing means the voltage to be compared by the second comparing means, PA1 whereby an alarm signal indicating as to whether the supply voltage is normal or abnormal is obtained at an output of the second comparing means. PA1 a high and a low potential power supply line; PA1 three comparing means having threshold values at the supply voltages being respectively V.sub.1, V.sub.2 and V.sub.3 (V.sub.1 &lt;V.sub.2 &lt;V.sub.3); PA1 a reference voltage source connected at its one side to the low potential power supply line and at the other side commonly to the three comparing means; and PA1 a logical circuit means receiving the respective outputs from the three comparing means and producing a first level output when the supply voltage is below V.sub.1 or between V.sub.2 and V.sub.3 and a second level when the same is between V.sub.1 and V.sub.2 or over V.sub.3, PA1 whereby whether the supply voltage is normal or abnormal is determined.
However, in either of the above two cases, a particular terminal is necessary either for the external setting or for the selective use of the monitoring circuits. As such a terminal cannot be used for other purposes, there is a disadvantage that other functions are prevented from being incorporated without an increase in the number of terminals.